1. Field of the Invention
The present invention relates to an optical scanning apparatus used for a copying machine and a laser printer.
2. Description of the Related Art
Conventionally, there have been widely utilized a digital copying machine and a printer, wherein electrolatent images are formed by irradiating electrically-charged photosensitive bodies with light beams modulated corresponding to image information, and the images are obtained through electrophotographic processes such as developing, transferring and fixing.
Similarly, a full-color copying machine and a color printer are widely utilized, wherein image signals corresponding to yellow (Y), magenta (M), cyan (C) and black (K) undergo charging, exposing and developing, and a full-color image is formed by superposing and transferring these color images.
Over the recent years, this type of full-color image forming apparatus has taken a widespread so-called tandem system, wherein image forming portions corresponding to the developing colors (Y, M, C, K) are arranged in series, and the full-color image is formed via one path in a way that sequentially superposes the transferred images.
One example of the tandem system is a multi-beam scanning apparatus (refer to Japanese Patent Application Laid-Open No. 2002-323668), wherein a plurality of light beams is arranged on a side-by-side basis in a sub-scanning direction and is made incident on mirror surfaces of a deflection mirror to be rotated, wherein a plurality of scanned surfaces is main-scanned by the plurality of light beams deflected by the deflection mirror. Note that this optical scanning apparatus employs a so-called under-field optical system in which a width, in the main-scanning direction, of the light beams incident on the deflection mirror is set narrower than a width, in the main-scanning direction, of one surface of the deflection mirror.
The optical scanning apparatus discussed in Japanese Patent Application Laid-Open No. 2002-323668 has a configuration that a plurality of cylindrical lenses through which the plurality of light beams incident on the deflection mirror passes are arranged side by side in the sub-scanning direction (vertical direction). In the under-field optical system, however, if the cylindrical lenses arranged side by side in the sub-scanning direction are not arranged with high accuracy in the main-scanning direction, such a problem arises that a scanning start position, in the main-scanning direction, of the light beam scanning over the scanned surface deviates between the plurality of light beams. If a color image is formed by use of the light beams having the deviation in their scanning start positions in the main-scanning direction, the color image results in an image with a color deviation in the main-scanning direction.
By the way, improvement of a recording speed (the number of output sheets per unit time) has increasingly demanded of the image forming apparatus over the recent years. In the printer and the copying machine, the increase in the number of output sheets per unit time must involve increasing a scan speed of the light beams over the photosensitive drums. A scan speed increasing method is exemplified by a method of increasing a rotating speed of a rotary polygon mirror and a method of providing a multi-beam system using a plurality of light sources. An over-field optical system capable of obtaining an increased number of reflection surfaces while restraining a diameter of the rotary polygon mirror, is known as one of the scan speed increasing techniques. The over-field optical system has a characteristic that a width, in the main-scanning direction, of the light beams incident on the rotary polygon mirror is larger than a width, in the main-scanning direction, of one surface of the rotary polygon mirror.
One of the over-field type optical scanning apparatuses is an apparatus including lenses each having power in only the main-scanning direction, wherein the width, in the main-scanning direction, of the light beams emitted from the laser light source is set large (refer to Japanese Patent Application Laid-Open No. 2004-020607).
If the construction that the plurality of light beams arranged side by side in the sub-scanning direction main-scans over the plurality of scanned surfaces through the single deflection mirror, is applied to the over-field optical system, however, the optical elements provided between the plurality of light sources and the deflection mirror are required to be positioned with the high accuracy in the main-scanning direction between the incident optical system elements (e.g., cylindrical lenses) each having the same optical characteristic. If the incident optical system elements are not positioned with the high accuracy in the main-scanning direction, it follows that a distribution of the light quantity used for the plurality of light beams to scan over the scanned surface in the main-scanning direction, differs. If the color image is formed by use of the light beams each having the different light quantity distribution, color densities of the images assuming developing colors (yellow, magenta, cyan, black) differ, and the color image formed by superposing these respective images does not obtain desired coloration.
As described above, in the case of employing either the under-field optical system or the over-field optical system, unless the plurality of incidence optical system elements arranged in the side-by-side relationship in the sub-scanning direction and having the same optical characteristic are arranged with high accuracy so as not to cause a relative positional difference in the main-scanning direction, a quality of the obtained image might deteriorate. Especially in the incidence optical system elements arranged in positions proximal to the light sources, if the relative positional difference exists between the plurality of incidence optical system elements in the main-scanning direction, the image quality is affected much into the deterioration corresponding to how long a distance at which the light penetrating the incidence optical system elements reaches the scanned surface (of a photosensitive body) is.